Do not consider unknown runtime type incompatible for consistency

Author: sterliakov

mypy/stubtest.py

@@ -1268,18 +1268,20 @@ def verify_var(
             runtime_type = get_mypy_type_of_runtime_value(runtime.value)
             if runtime_type is not None and is_subtype_helper(runtime_type, stub.type):
                 should_error = False
-            # We always allow setting the stub value to ...
+            # We always allow setting the stub value to Ellipsis (...), but use
+            # _value_ type as a fallback if given. If a member is ... and _value_
+            # type is given, all runtime types should be assignable to _value_.
             proper_type = mypy.types.get_proper_type(stub.type)
             if (
                 isinstance(proper_type, mypy.types.Instance)
                 and proper_type.type.fullname in mypy.types.ELLIPSIS_TYPE_NAMES
             ):
                 value_t = stub.info.get("_value_")
-                if value_t is None or value_t.type is None:
+                if value_t is None or value_t.type is None or runtime_type is None:
                     should_error = False
-                elif runtime_type is not None and is_subtype_helper(runtime_type, value_t.type):
+                elif is_subtype_helper(runtime_type, value_t.type):
                     should_error = False
-                elif runtime_type is not None:
+                else:
                     note = " (incompatible '_value_')"
 
         if should_error: